Use the simulator under this link https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion-basics_en.html . In this extension activity, you will change the friction and observe what happens to the net force and how this affects the acceleration.

Use the "Acceleration" module of this simulation and click on the "Reset" button to clear all values. This sets the friction slider in the middle. Apply a 50 N force to the wooden crate.

1. Check the "Values" box. What is the friction force? Is the object moving?

2. Add another 50 N. What is the friction force? Is the object moving?

3. Add another 50 N for a total of 150 N applied force. Is the object moving? What are the fric- tion force and the net force? Remember that net force is the overall force; it is determined by subtracting the friction force from the applied force.

4. Using this net force, predict the acceleration of the box using the equation that relates force, mass and acceleration.

5. Click the box next to "Acceleration" to check your work. Did you do your calculation correctly? If not, redo the calculation.

6. Predict the acceleration for two crates on top of each other and then test it. Place the second crate above the first and then add force 50 N at a time until the crates move. Using the new net force and mass, calculate the acceleration.

7. Clear the screen and click on "Sum of Forces," "Values," and "Masses." Decrease the friction slider to an amount below the default. Add 150 N of applied force to the wooden crate. What happens to the net force compared to when the friction slider was halfway? Calculate the new acceleration.

8. Predict what would happen to the object with the same applied force if the friction were larger.

8. Predict what would happen to the object with the same applied force if the friction were larger.